1. Field of the Invention
Embodiments described herein relate generally to a magnetic resonance imaging apparatus and a load calculation method of a gradient magnetic field generation system.
2. Description of the Related Art
MRI is an imaging method which magnetically excites nuclear spin of an object (a patient) set in a static magnetic field with an RF pulse having the Larmor frequency and reconstructs an image based on MR signals generated due to the excitation. The aforementioned MRI means magnetic resonance imaging, the RF pulse means a radio frequency pulse, and the MR signal means a nuclear magnetic resonance signal.
A gradient magnetic field generation system in an MRI apparatus includes a gradient magnetic field coil which adds spatial positional information to MR signals by applying a gradient magnetic field in an imaging space where an object is set. This gradient magnetic field coil produces heat by being provided with pulse electric current during imaging. A gradient magnetic field generation system has various limitations in terms of the total upper limit of electric power, the respective upper limits of electric power in each channel and the like, and does not have enough ability to endure the maximum electric current in every channel (X axis direction, Y axis direction and Z axis direction) concurrently.
However, in conventional technology, it is difficult to precisely estimate application limits of a gradient magnetic field generation system in terms of electric power according to an imaging sequence.
Therefore, in prior art, initial rise characteristics of a gradient magnetic field coil are calculated by using an ohmic value at “the maximum temperature allowable for the gradient magnetic field coil”, imaging conditions are determined in this calculated range, and then the gradient magnetic field coil is driven (see, for example, Patent Document 1).
Although the initial rise characteristics in the aforementioned case are under the worst conditions, the temperature of a gradient magnetic field coil rarely reaches the maximum temperature in actual status of use.
As just described, a gradient magnetic field generation system is safely driven under control of keeping a sufficient margin between actual supplied amount of electric current and the application limit value. That is, the supplied amount of electric current to a gradient magnetic field generation system is controlled so as to surely fall below its application limit value.    [Patent Document 1] Japanese Patent Application Laid-open (KOKAI) Publication No. H08-56917
In the aforementioned conventional technology, though there is an enough margin from its application limit, a gradient magnetic field generation system is sometimes driven more safely than its application limit. If there was an enough margin up to the application limit of a gradient magnetic field generation system, imaging could be performed under more optimized conditions by increasing a slice number by the value corresponding to the margin, for example.
Thus, technology of accurately estimating electric load on a gradient magnetic field generation system in MRI depending on an imaging conditions has been desired in order to perform imaging under more optimized conditions.